A conventional this kind of water turbine has a formation that a long rotor shaft is provided in an axis direction in a long water conduit pipe, pluralities of rotors are arranged in a line on this rotor shaft, and an underwater hydraulic power generator is driven by rotation of the rotor shaft (see JP 2000-9012 A).
As a conventional connecting structure for linearly connecting two male screw shafts, the followings are exemplified. A small-diameter screw hole is provided on one of end surfaces opposing each other of two male screw shafts arranged linearly, a small-diameter screw shaft is provided on the other end surface, and they are screwed each other (see JP S13-3817 U). Each end surface of two male screw shafts is brought into contact, and a tubular coupler whose inner surface is provided with a female screw thread having the same pitch as that of a male screw thread of both male screw shafts is screwed on an abutted part of them to cover the abutted part (see JP 2014-15728 A). Each end surface of two male screw shafts is brought into contact, an abutted part of them is held by a pair of halved bodies of a cylindrical screw whose inner surface is provided with a female screw thread having the same pitch as that of a male screw thread of both male screw shafts, and a nut is screwed to a male screw thread provided on an outer peripheral surface of the cylindrical screw (see JP 2001-220859 A).